1. Field of the Invention
This invention relates generally to an active matrix liquid crystal display device, and more particularly to a technique of preventing charge from remaining in liquid crystal picture elements.
2. Description of the Related Art
Conventional liquid crystal display devices of the active matrix type have such a general construction as shown in FIG. 6. Referring to FIG. 6, the liquid crystal display device shown includes a liquid crystal layer 103 held between a driving circuit board 101 and an opposing circuit board 102 which are disposed opposite to each other with a predetermined gap therebetween. Gate lines 104 and signal lines 105 are disposed in an intersecting relationship with each other in a matrix on a surface of the driving circuit board 101, and picture element electrodes 106 are formed at individual intersecting points between the gate lines 104 and the signal lines 105. Also picture element transistors 107 are formed corresponding to the individual picture element electrodes 106. The gate electrodes of the picture element transistors 107 are connected to corresponding ones of the gate lines 104, and the drain electrodes of the picture element transistors 107 are connected to corresponding ones of the picture element electrodes 106 while the source electrodes of the picture element transistors 107 are connected to corresponding ones of the signal lines 105. Meanwhile, common electrodes 108 and a color filter layer 109 are provided on an inner face of the opposing circuit board 102. Liquid crystal picture elements are defined in a matrix by the liquid crystal layer 103 held between the common electrodes 108 and the picture element electrodes 106 opposite to each other. The two circuit boards 101 and 102 are adhered to each other, and polarization plates 110 and 111 are adhered to outer surfaces of the circuit boards 101 and 102.
FIG. 7 is a circuit diagram of an equivalent circuit for a liquid crystal picture element in such a conventional liquid crystal display device as shown in FIG. 6. Referring to FIG. 7, the gate electrode of a picture element transistor Tr is connected to a gate line Y so that a gate pulse V.sub.G is applied to the gate electrode for a predetermined selection period. The source electrode of the picture element transistor Tr is connected to a signal line X so that an image signal Vsig, which reverses after each one field period or one horizontal scanning period with reference to a predetermined reference potential, is supplied to the source electrode. The drain electrode of the picture element transistor Tr is connected to a corresponding picture element electrode E. A liquid picture element PXL is defined by the liquid crystal layer held between the picture element electrode E and a common electrode COM opposite to the picture element electrode E. A predetermined reference potential Vcom is applied to the common electrode COM. It is to be noted that the liquid crystal picture element PXL is a capacitive load and normally has an auxiliary capacitance in addition to the capacitance of the liquid crystal.
FIG. 8 illustrates operation of the liquid crystal picture element shown in FIG. 7. Referring to FIG. 8, each time a gate pulse V.sub.G is applied, the picture element transistor Tr conducts so that an image signal Vsig is written into the liquid crystal picture element. After the gate pulse V.sub.G disappears, the image signal Vsig is held in the liquid crystal picture element. When another gate pulse V.sub.G is applied at next selection timing, another image signal Vsig reversed with respect to a predetermined center potential Vref is written into the liquid crystal picture element. In principle, in order to effect ac driving, the reference potential Vcom to be applied to the common electrode COM is set so as to coincide with the center potential Vref of the image signal Vsig. However, an actual liquid crystal picture element potential Vp must necessarily be optimized by adjusting the reference potential Vcom since actually it shifts downwardly from the level of the image signal Vsig. As seen from FIG. 8, when a gate pulse V.sub.G falls, a voltage drop is produced by capacitive coupling between the gate electrode and the drain electrode of the picture element transistor Tr, and consequently, the picture element potential Vp drops by .DELTA.V.sub.A on the positive polarity side and drops by .DELTA.V.sub.B on the negative polarity side. The reference potential Vcom optimized taking the voltage drops into consideration is given by Vref-(.DELTA.V.sub.A +.DELTA.V.sub.B).
FIG. 9 illustrates the difference between the optimum values of the reference potential Vcom when an ordinary operation is performed and when a referring operation is performed. The data of the graph were obtained from a measurement conducted for a large number of samples in order to detect a dispersion of the reference potential Vcom. The optimum reference potential in an ordinary operation is represented by VcomN. The optimum reference potential VcomN is obtained by subtracting the voltage drop caused by capacitive coupling from the center potential Vref of the image signal Vsig as described above with reference to FIG. 8. Meanwhile, in a referring operation, the image signals Vsig are supplied while all of the picture element transistors are always in a conducting state. In this instance, since no gate pulse falls, no voltage drop by capacitive coupling is produced. Consequently, the optimum reference potential VcomH substantially coincides with the center potential Vref of the image signals Vsig. Accordingly, for each sample, the voltage drop by capacitive coupling is given by VcomH-VcomN=.DELTA.Vcom. As apparently seen from the graph of FIG. 9, the voltage drop ranges from 0.3 to 0.4 V with all of the samples and the dispersion is very small. Accordingly, it is comparatively easy to set, for individual liquid crystal display devices of the active matrix type, an optimum value for the reference potential Vcom which is compensated for by a substantially fixed voltage drop.
However, the conventional liquid crystal display device of the active matrix type has a problem to be solved in that, since residual of charge actually occurs with liquid crystal picture elements, the optimum value of the reference potential Vcom set once undergoes an apparent variation. This will be described briefly with reference to FIG. 10. If a liquid crystal picture element PXL is driven continuously, then charge is accumulated in an interface between the common electrode COM and an orientation film, another interface between the picture element electrode E and the orientation film and so forth to make a charge residual condition. If ac driving of the liquid crystal picture element is stopped once in such a charge residual condition, then this results in connection of an imaginary dc power source VDC to the liquid crystal picture element PXL. Accordingly, when ac driving of the liquid crystal picture element PXL is started again, an offset of the dc voltage VDC is added to the optimum value of the reference potential Vcom set precedently so that the optimum value of the reference potential Vcom is varied apparently.
This will be described using detailed values with reference to FIG. 11. It is assumed that an image signal Vsig of 6.+-.2 V is supplied to the signal line X. If a voltage drop caused by the capacitive coupling described above is not taken into consideration, then the reference potential Vcom is set to 6 V so that it may coincide with the central potential of the image signal Vsig. It is also assumed that the liquid crystal picture element PXL is driven continuously, and consequently, residual of charge occurs with the liquid crystal picture element PXL so that a dc offset VDC is added to the reference potential Vcom. If the liquid crystal picture element is driven in this condition, then the effective reference potential Vcom changes to 1 V+6 V=7 V. In this condition, complete ac driving cannot be performed. In other words, as a result of the residual of charge, the optimum reference potential Vcom apparently varies from 6 V to 5 V. Since such dc offset amount relies upon the time of continuous driving and also upon the magnitude of the image signal and so forth, a dispersion of the optimum reference potential Vcom occurs even within a single panel.
FIG. 12 illustrates an example of the variation of the optimum value of the reference potential Vcom with respect to time. In order to obtain the measurement data of FIG. 12, ten samples were prepared. Each of the samples was driven by ac which was reversed for each one horizontal period, and the image signal Vsig was set to 6 V fixed. Further, in order to accelerate residual charge, the potential at the common electrode was set to 1.5 V for a half of the samples and to 10.5 V for the remaining half. The optimum value of the reference potential Vcom was measured at an initial stage and after lapse of time of one hour to examine the variation of the same with respect to time. The optimum value of the reference potential Vcom of 5.70 to 5.79 V at the initial stage varied between 5.45 V and 6.03 V after lapse of time of one hour. Although actually such an extreme charge residual condition may not be applicable, residual charge actually occurs with a liquid crystal picture element and the optimum value of the reference potential Vcom varies as a result of continuous driving of the liquid crystal picture element for a long period of time. Further, a dispersion in amount of residual charge occurs even within a single panel.
The conventional liquid crystal display device of the active matrix type thus has a problem to be solved in that, since the optimum value of the reference potential Vcom varies, a seizure of the screen or a residual image occurs, which deteriorates the picture quality remarkably. Further, in addition to seizure, deterioration of the picture quality such as reduction in contrast, flickering and so forth are a problem. Since the variation of the optimum value of the reference potential Vcom with respect to time by residual of charge cannot be compensated for at an initial stage, this is a serious problem to be solved for quality assurance.